Generally, thermal reflow is one of the techniques most widely employed in a process for forming a micro-lens array of an image sensor. Thermal reflow applies heat to a photo-resist pattern to reflow it to thus obtain a lens form having a desired curvature.
Use of thermal reflow, however, when a micro-lens in a fluid state comes into contact with a neighbor micro-lens during the reflow process, the micro-lenses in contact tend to conglomerate due to surface tension of the fluid. This makes the micro-lens abruptly bridged with the neighbor micro-lens and the curvature of the bridged micro-lens distorted, which results in a defective micro-lens. Accordingly, the use of the thermal reflow actually makes it difficult to form a perfect zero-gap micro-lens, i.e., without a gap between the micro-lens itself and the neighbor micro-lenses.
FIGS. 1A and 1B illustrate a sequential process of a method for solving the problems in forming the micro-lens and a top-down view of each process in accordance with the related art, respectively.
As illustrated in FIG. 1A, in the related art, in order to solve the bridge problem with the neighboring micro-lens arising in the micro-lens forming process, a photo resist pattern of a first micro-lens is formed and thermally reflowed, and a photo resist pattern of a second micro-lens is then formed in an empty space on a semiconductor substrate and then thermally reflowed, rather than forming the neighboring micro-lens at the same time. Namely, the micro-lens is formed through a 2-step micro-lens forming process or a dual micro-lens forming process.
In such a 2-step micro-lens forming process, the micro-lenses neighboring in a horizontal or vertical direction are formed separately two times, reducing an occurrence of a lens bridge, whereby a perfect zero-gap can be formed.
As illustrated in FIG. 1B, however, when the distance “a” from a micro-lens neighboring in a diagonal direction is zero, a lens bridge is also generated in the diagonal direction. There is a limitation, therefore, in reducing the diameter of the dead zone such that it is less than a certain distance. Meaning, in the general 2-step micro-lens forming process, an adjustable diameter of the dead zone is about 0 nm to 300 nm, which is constant regardless of a pixel pitch. Thus, when the ratio between the pixel area and the dead zone area is taken into consideration, additional improvement is required for pixels having a size of less than 1.4 μm.
Meanwhile, when the size of the pixel is reduced to be 1.2 μm or smaller, optimum lens curvatures of respective red, green, and blue colors should each be different. The existing 2-step micro-lens forming process, however, merely divides the thermal reflow into two steps to simply perform the respective steps separately, and thus, is incapable of forming the respective pixel colors with different curvatures. Accordingly, using this technique it is difficult to achieve optimization due to an increase in the pixel-tech.
In addition, in the above-noted 2-step micro-lens forming process, the lens shape is formed using thermal reflow in both first and second steps of the micro-lens forming process. In such a case, different optimal conditions need to be sought depending on pixel sizes in the thermal reflow. Consequently, there is a problem in that whenever the pixel size is reduced, the optimization process needs to be performed several times, respectively.
As illustrated in FIG. 2, in order to overcome the limitation of the existing thermal reflow, a micro-lens forming process using a gray-tone mask 200 derived from an MEMS (micro electro mechanical systems) process has recently come into prominence. In the micro-lens forming process using the gray-tone mask 200, a mask pattern is formed as if a dot painting was drawn with dots smaller than resolution, to allow the intensity of transmitted light to be continuously changed depending on the density of dots. A desired curvature is thus obtained only with photolithography.
When a micro-lens array is formed by using the gray-tone mask, a desired curvature can be freely formed for each color since the gray-tone mask is mainly dedicated for forming a lens of a pitch of tens of μm of MEMS. In a case of a micro-lens array for an image sensor whose pixel size is merely 1 μm to 2 μm, however, a gap space profile formed between neighboring lenses need to be sharply changed within a distance of about 0.1 μm to 0.2 μm.
The degree of the sharpness of the gap space profile, however, is determined depending on photolithography resolution. As illustrated in FIGS. 3A through 3C, consequently, in a case of photo-resist for a micro-lens using an i-line wavelength, in the micro-lens forming process using the gray-tone mask 310, and an SEM (scanning electron microscope) photograph, a severe gap space rounding 300 is formed to reduce effective curvature of the micro-lens and increase the size of the dead zone at which four lenses are in contact.